drm/fsl-dcu: avoid disabling pixel clock twice on suspend

With commit 0a70c998 ("drm/fsl-dcu: enable pixel clock when
enabling CRTC") the pixel clock is controlled by the CRTC code.
Disabling the pixel clock in suspend leads to a warning due to
the second clk_disable_unprepare call:
  WARNING: CPU: 0 PID: 359 at drivers/clk/clk.c:594 clk_core_disable+0x8c/0x90

Remove clk_disable_unprepare call for pixel clock to avoid
unbalanced clock disable on suspend.

Fixes: 0a70c998

 ("drm/fsl-dcu: enable pixel clock when enabling CRTC")
Signed-off-by: Stefan Agner <stefan@agner.ch>